A foam of a polyolefin resin composition has an excellent balance of properties and cost and is excellent in a recycling property of the resin. The foam of a polyolefin resin composition is thereby widely used for applications as heat insulating building materials, applications as automobile parts, applications as packaging cushion materials, etc. For example, for an application as heat insulating building materials, a foam board of a polypropylene resin or a polyethylene resin is applied to the inside of floor or wall of buildings. Since such a foam board has an excellent heat insulating property, it is widely used in markets.
Foam boards and foam sheets made of a polyolefin resin composition have already been studied variously. For example, Patent Document 1 discloses a closed cell foam of a polypropylene with a high foaming rate, which is in the form of a bundle of strands.
Further, Patent Document 2 discloses a method for producing a polyethylene resin foam having open cells. This production method comprises mixing a molten crosslinking polyethylene resin (a), a heat decomposable foaming agent and an organic peroxide and further a powder polypropylene resin (b) having a melting point higher by 20° C. than the crosslinking polyethylene resin and being at most 190° C., in a specific ratio and uniformly dispersing the powder polypropylene resin (b) in a non-molten state, followed by heating so that while the crosslinkable polyethylene resin (a) is cross-linked and foamed, and the polymer polypropylene resin (b) is melted.
Further, Patent Document 3 discloses an open cell foam of a polyolefin resin having a cell size of at least 1 mm and a method for its production. That is, the cell size is large at a certain level, and in some cases, by using a foam of a polyolefin resin composition on which holes are formed by a needle or the like, a sound absorption effect is aimed.
Further, Patent Document 4 discloses a method for producing an extruded propylene resin foam produced by heating a propylene resin so as to be in a molten state and kneading the molten propylene resin while applying a shearing stress, followed by extrusion-foaming from a die for forming. In this production method, a propylene resin is extrusion-foamed so that at the position where the cross section perpendicular to the flowing direction of the resin flowing path becomes the minimum in the vicinity of an exit of the die for extrusion, the pressure gradient (k) represented by a specific formula satisfies 50 MPa/m≦k≦800 MPa/m, and the decompression rate (v) represented by a specific formula satisfies 5 MPa/s≦v≦100 MPa/s. Patent Document 4 discloses an open cell foam wherein the total area of broken cell parts evaluated by a cross-sectional photograph of the foam thus obtained is at least 2% of the total area of the observed surface.
Patent Document 1: JPB2620968
Patent Document 2: JPB2673310
Patent Document 3: JP-A-2002-524636
Patent Document 4: WO2006/101142